Such structures are generally known in the art, for instance from documents US2008/0250746A1; US2010/0005751; WO03/002830; and U.S. Pat. No. 5,125,206. Examples of common elements of such structures are tension elements and rigid elements, such as rods and beam ties that are typically interconnected by interconnecting elements, e.g. by welding together the rigid elements or by joints and hinges. The rigid elements are generally known as being able to resist tension, compression and bending loads. Prior art also describes statically under-determined structures. An example of a two-dimensional (2D) statically under-determined structure is illustrated in FIGS. 1a through 1e herein and comprises rigid elements (1), i.e. rods, interconnected by interconnecting elements (2), i.e. hinges; no force is applied to this structure. FIG. 2 shows a statically under-determined structure as depicted in FIG. 1a in a situation when forces F (force may also be herein referred to interchangeably as load) are applied to the structure and, as a result, the structure is deformed under diagonal compression forces F; such high deformation is normally allowed by the interconnecting elements. A statically under-determined structure is thus typically an unfavorable structure, as it allows large deformation under load, without loading the tension, compression and/or bending resistance of the rigid elements. Similar considerations are also valid for a three-dimensional (3D) statically under-determined structure.
A statically determined structure known in the art is shown in FIG. 3 in relation to the statically under-determined structure of FIG. 1b. The addition of a tension element (3), i.e. a rod, usually prevents the elongation of the distance between the lower left and the upper right interconnecting elements (2), i.e. hinges. In this way, the high deformation due to the forces F is typically prevented, making the structure stabile (e.g. it does not fail) in all cases of in-plane loading, including loads applied to the structure in other directions than the direction of loads F illustrated in FIG. 3.
Statically over-determined structures typically contain more structural elements than strictly necessary to carry external loads, with the consequence that such a structure may be loaded by internal forces, even in the case that the said structure is not externally loaded. When applying external loads to such a structure, said external forces generally increase the internal forces. The sum of the internal forces and the external forces may be higher than the load capacity of an individual structural element and thus may cause premature failure of said element, resulting in subsequent premature failure of the entire structure. The load capacity is herein defined as the force applied to the structure or an element of the structure under which said structure fails to resist the applied loads. An example of statically over-determined structure known in the art is shown in FIG. 4 in relation to a statically under-determined structure of FIG. 1c. FIG 4 shows the addition of another tension element to the structure depicted in FIG. 3, i.e. the rod (4) to form a statically over-determined structure. Such a structure typically resists any loads applied to it. However, to be an effective structure, the length of the rod (4) should be generally identical to the distance between the upper left and lower right-hand side hinges, because this distance was already established by adding the rod (3), as shown in FIG. 3. In the case that rod (4) is longer or shorter than said distance, the rod and the structure would have to be deformed in order to fit said distance. Such a deformation typically requires applying forces on the structure, including on the rigid elements. Such forces are normally undesirable internal forces that are still imposed on the structure when the structure is in service.
In prior art, mechanical devices, such as hydraulic devices are generally used to reduce internal forces on elements in statically determined or over-determined structures that could lead to premature failure. These mechanical devices commonly relieve internal loads on elements by changing the effective length of the elements. Prior art also discloses tension elements in such structures that comprise different materials used with the aim to stabilize internal loads. Examples of such materials include steel, polyester fibers, polyethylene fibers, aramid fibers. However, steel has heavy weight and is corrosive; in addition, in structures that use steel, initial length differences between different tension elements (e.g. tendons in marine platforms) need to be cancelled out by actively adjusting the height of the end fixtures of the tension elements by using expensive hydraulic devices. Polyester fibers show lower strength and therefore very thick tension elements, such as cables containing polyesters are needed, resulting in operational problems. Aramid fibers exhibit low abrasion resistance and lack chemical resistance especially when used in alkaline environments, such as in salty see water. Polyethylene fibers, particularly ultrahigh molecular weight polyethylene fibers (e.g. Dyneema® and Spectra®) exhibit excess minimum creep, that leads to creep failure of the structure. Such UHMWPE materials were for instance described by M. P. Vlasblom and R. L. M. Bosman in Predicting the Creep Lifetime of HMPE Mooring Rope Applications, published in the abstracts of Ocean 2006 Conference, Boston Mass., September 2006, Publisher: IEEE, Print ISBN: 1-4244-0114-3.
Furthermore, structures are generally designed for very long time service load and during that time varying loads generally occur. The magnitude of the loads may be a statistical distribution, e.g. loads due to wind gusts in storms. In addition, the design of the structures is typically made by adopting a very low chance of exceeding the design strength level of the structure during the life time, e.g. for an offshore platform it may be adopted that a chance is only e.g. 1/1000 that a storm occurs during an operational life time of the structure of e.g. 20 years, with a storm intensity being so high such that the design load levels that the structure resists are surpassed. The statistical expectation for such a high loading may be around the midlife of such a structure, the chance that it occurs in the first weeks of the life time of such a structure being extremely low. Moreover, present weather prediction models generally allow erecting such a structure at a timing that such a storm is typically not expected. An attractive strategy may be to erect such a platform at the time when bad weather conditions are very rare (e.g. in the summer). Consequently, a temporary strength reduction during the first week(s) or even months after erecting the structure is accepted, but it is desired that strength reduction is only temporary.
The objective of the present invention is therefore to provide a structure that avoids the disadvantages of the prior art, particularly to provide a structure that is very stable, allows internal loads to be reduced and thus avoids premature failure when internal and/or external forces are applied to said structure, without the need of using expensive mechanical devices and in the same time may be light weighted and have high mechanical strength.
This objective is surprisingly achieved by a structure comprising rigid elements connected together by interconnecting elements in such a way to form a statically determined structure or a statically over-determined structure, wherein said structure comprises at least one tension element comprising polymeric fibers having a stabilizing creep of at least 0.3% and at most 10% and a minimum creep rate lower than 1×10−5% per second, said stabilizing creep and minimum creep being measured at a tension of 900 MPa and a temperature of 30° C. Even if the structure according to the present invention may undergo an accepted temporary strength reduction during the first week(s) after erecting, the strength reduction of the structure according to the present invention is surprisingly only temporary and thus said structure will show improved safety during the majority of its life time.
It is true that document DE102008005051B3 discloses such a structure. In particular, this document discloses a cable structure for cable tensioned space framework, the structure having a sleeve rotatably supported and being fixed around a longitudinal middle axis in a node element so that cable is stressed longitudinal and transversal to carrying direction. However, the structures described in this document comprise cables made of polymeric fibers (i.e. Kevlar®, i.e. a para-aramid synthetic fiber) that are different than the polymeric fibers according of the present invention and therefore the structures described in this document will prematurely fail when loads are applied to them. Document WO2014/210026A2 describes structures for tethering a subsea blowout preventer comprising anchors, tensioning systems and tensioning members. The tensioning members described in this document can include chains, wire rope or Dyneema® rope available from DSM Dyneema LLC of Stanley, N.C. USA. Said Dyneema® rope was made of fibers that have different properties than the fibers according to the present invention and therefore the structures described in this document will prematurely fail when loads are applied to them.